1. Field of the Invention
The invention relates to a method of producing a material of which a fuel cell electrode preferably used as an electrode (in particular, a cathode) for a polymer electrolyte fuel cell is composed. The invention relates more particularly to a method of producing a material of which an electrode used for a fuel cell is composed, the material including non-carbon electrically conductive oxide supports as electron-conductive supports.
2. Description of the Related Art
Since a fuel cell is able to effectively convert chemical energy of hydrogen into electric energy, it is expected that an electric power generation system making use of a fuel cell is broadly used. Among fuel cells, a polymer electrolyte fuel cell (PEFC) including a solid polymer membrane as a membrane electorolyte is able to work at a relatively low temperature, specifically, at about 80 degrees centigrade, and hence, is expected to be used as a small-sized fuel cell for houses, for instance.
PEFC comprises an anode (a fuel electrode) attached to one of surfaces of a solid polymer membrane electorolyte, and a cathode (an air electrode) attached to the other. Supplying hydrogen as fuel to the anode, and air (oxygen) to the cathode, electric power is generated in accordance with the following electrochemical reactions.Reaction in Anode: 2H2→4H++4e−  (Reaction 1)Reaction in Cathode: O2+4H++4e−→H2O  (Reaction 2)Overall Reaction: 2H2+O2→2H2O
As a material of which an electrode used for PEFC is composed, presently broadly used are a material comprising particles of noble metal such as platinum (Pt) and Ruthenium (Ru), and a particle-shaped or fiber-shaped carbon material on which the nobel metal particles are dispersed (for instance, see Japanese Patent Application Publication No. 2005-87993 and Japanese Patent No. 368364). The noble metal particles provide electrochemical catalytic activities, specifically, reduction of oxygen (and oxidation of hydrogen), and the carbon material works as supports for loading the noble metal particles thereon. Furthermore, since the carbon material has high electron-conductivity, the carbon material acts as a path through which electrons run in the above-mentioned Reactions 1 and 2.
Since a membrane electorolyte used in PEFC is acidic (pH is 0 to 3), a material of which electrodes in PEFC are composed is used in acidic atmosphere. While a fuel cell normally works, a cell voltage is in the range of 0.4 to 1.0 V. FIG. 11 is the Pourbaix diagram illustrating a relation between a voltage and pH in a carbon-water (C—H2O) system. Illustrating situations of a cathode and an anode of PEFC working under the above-mentioned conditions, it is understood that they are in an area in which carbon acting as a support in a cathode is decomposed as carbon dioxide (CO2). Accordingly, as reported in the non-patent reference 1, there occurs in a cathode a reaction in which a carbon material used as a support is electrochemically oxidized or decomposed into CO2.C+2H2O→CO2+4H++4e−  (Reaction 3)
Not only in a cathode, but also in an anode, if fuel gas became in short at an initial stage of operation, there would occur reduction in a voltage and/or polarization in a concentration, resulting in that a voltage locally turns to a voltage which is opposite to a normal voltage, and there occurs electrochemical oxidation decomposition reaction in carbon.
Electrochemical oxidation of a carbon support is a problem in particular when PEFC is driven for a long time. Specifically, the noble metal particles on carbon supports are fallen due to oxidative decomposition of carbon supports. As a result, the electrode performance is degraded.
Further, due to local reaction heat brought by oxidation reaction of carbon, a polymer membrane electorolyte is molten to break with the result of occurrence of cross-leakage. Furthermore, if the local reaction heat spreads there around, an entire stack of a fuel cell might be burned. Thus, it is desired to develop a material of which an electrode used for a fuel cell is composed, which is stable under conditions in which a fuel cell is driven, and including supports composed of a non-carbon material having sufficient electrode performances.
The related art, L. M. Roen et al., “Electrocatalytic Corrosion of Carbon Support in PEMFC Cathodes”, Electrochemical and Solid-State Letters, 2004, Vol. 7(1), A19-A22, discloses a material of which an electrode used for a fuel cell is composed, comprising supports composed of electrically conductive metal oxide in place of a carbon material, and noble metal catalyst is loaded on the supports. Specifically, the related art discloses in the embodiment thereof that even after a cycle of driving and stopping a fuel cell were repeated 500 times under general conditions for driving a fuel cell in a fuel cell (PEFC) including electrodes composed of a material comprising supports composed of electrically conductive metal oxide such as tin-doped indium oxide, and Pt particles loaded on the supports at about 50 mass %, a cell voltage was hardly degrated.
In the material disclosed in the related art, “Electrocatalytic Corrosion of Carbon Support in PEMFC Cathodes”, Pt is loaded on surfaces of powders of electrically conductive oxide by dispersing powders of the above-mentioned electrically conductive oxide in an aqueous solution of chloroplatinic acid, and adding sodium citrate as a reducing agent thereinto for reflux. Though the related art describes that Pt particles having a small diameter and having high dispersity ability are formed on surfaces of powders composed of electrically conductive oxide, the disclosed material cannot provide performances satisfied as a fuel cell electrode.
In view of the above-mentioned current state, it is an object of the present invention to provide a method of producing a material of which an electrode used for a fuel cell is composed, which includes supports composed of electrically conductive oxide support and having excellent electrochemical catalytic activity, and high durability